The prior art includes many teachings of full color electrophotographic (EP) printer configurations. Many color EP printers employ a four-pass configuration wherein four developer modules are arrayed along a photoreceptor surface. The developer modules are allocated to the deposition of cyan, yellow, magenta, and black toners onto the moving photoreceptor surface. A charging station uniformly sensitizes the photoreceptor surface. An exposure station selectively discharges the photoreceptor surface in accordance with respective color plane image data, The photoreceptor surface is then passed over the developer modules, with one developer module being brought into engagement with the photoreceptor surface to allow development of one color of the exposed image. The developed photoreceptor image then experiences a full rotation, is again exposed in accord with next color plane data and the re-exposed image is again developed, using the next color. The procedure continues until four passes have occurred and the entire full color image is present on the photoreceptor. An image transfer action then occurs whereby the color-toned image is transferred to a sheet which then issues from the printer. U.S. Pat. No. 5,314,774 to Camis discloses such a system and employs a plurality of dry powder, color toner developer modules to enable the operation of a four-pass color printer. The Camis apparatus employs a non-magnetic toner which enables the use of dot-on-dot image development.
U.S. Pat. No. 5,300,990 to Thompson illustrates a liquid EP printer developer module and further describes (see FIG. 3) that such developer modules can be positioned side-by-side beneath a web-photoreceptor. The Thompson patent does not disclose whether the liquid EP system is single pass or four pass. Once the image in the Thompson system is fully developed on the photoreceptor surface, it is transferred to a sheet of paper or to an intermediate transfer medium.
U.S. Pat. No. 5,016,062 to Rapkin discloses a multicolor EP printer which includes four secondary imaging drums that are positioned along the path of an endless web. In accordance with the multi-color image to be produced, each drum is appropriately exposed in accordance with data from a single color plane and a paper sheet is passed in contact therewith via the endless web to enable toner transfer. After the sheet has contacted all of the secondary imaging drums, it contains a full color image. A similar system is shown in U.S. Pat. No. 4,905,047 to Ariyama, however, the Ariyama system employs a liquid toner to achieve the imaging of the respective secondary drums. U.S. Pat. No. 4,788,574 to Matsumoto et al. also discloses a four-drum/conveyor belt developer system for an in-line color printer.
To increase the speed of EP apparatus, the prior art has suggested single-pass color-printers. European Patent 0 599 296 to Fukuchi et al. illustrates a single pass color copier which includes a four plane memory for storing yellow, magenta, cyan and black pixel data. In one embodiment, Fukuchi et al. use a web photoreceptor having a plurality of liquid toner developer modules arrayed along one surface. Between each developer module, a laser beam images the web photoreceptor in accordance with a particular color plane's pixel data. Immediately after each imaging action, a development occurs in accordance with the charge states on the web photoreceptor. Next, the web photoreceptor is again charged and developed in accordance with a next color plane's image data. The procedure continues until all four image planes have been exposed and developed, at which point the image is transferred to a paper sheet. Fukuchi et al. employ powder toners to achieve their individual color toning actions.
U.S. Pat. No. 4,599,285 to Haneda et al. discloses an EP apparatus wherein plural developers are positioned along a photoreceptor web, with each developer module employing a two-component powder toner. Electrostatic recording heads are positioned between the individual developer modules to allow a writing of pixel charge states on the photoreceptor web in accordance with particular color plane data.
While it is known that the speed of a single pass color EP printer can be made four times faster than a four-pass print architecture, single-pass EP color printers present a number of problems. It is difficult to assure proper registration of subsequent image color planes if the photoreceptor web is subject to speed variations as a result of engagement and disengagement of developer modules. Web speed variations cause a "banding" in the image and are to be avoided. In EP color printers that employ liquid toners, a line of fluid is created by surface tension of the toner carrier when a wetted roller or blade is removed from the surface of the photoreceptor. Means are generally provided to remove the "drip" line so as to prevent it from contaminating the system. Further, complex apparatus is required to enable engagement and disengagement of developer modules and transfer rollers from the photoreceptor web. The speed of the EP printer is further dependent upon the time it takes to disengage a developer module and engage a next developer, etc.
Accordingly, it is an object of this invention to provide a single-pass, full color EP printer exhibiting an improved architecture and speed of operation.
It is another object of this invention to provide an improved full-color EP printer that employs liquid toner developer modules, but avoids drip lines on the photoreceptor.
It is a further object of this invention to provide an improved full color EP printer wherein mechanisms to engage and disengage developer modules are avoided.